1. Field of the Invention
In steam turbines it is necessary to provide a sealed passage for steam between separate casings. Since the casings have different rates and magnitudes of thermal expansion, the seals must be capable of accepting vertical, axial, and transverse differential expansion relative to the axis of the turbine shaft, without permitting significant leakage.
2. Description of the Related Art
A common method of sealing employs a stack of piston rings that surround a pipe or snout that extends between the casings. The stack of piston rings alternate in size, with somewhat smaller rings that fit tightly to the snout being held between larger piston rings that are held in the bore of the casing such that the outer surface of the rings seal tightly to it. This method permits sliding motion of the pipe through the piston rings while maintaining a seal. It also permits transverse and axial motion of the pipe by sliding the small piston rings relative to the large piston rings, still maintaining a tight seal between the opposing large and small piston ring surfaces while the inner surface of the small piston rings and the outer surface of the large rings continue to seal tightly with the pipe and casing surfaces, respectively.
The rings must be fabricated from a high strength super alloy and must also have a coefficient of thermal expansion that is consistent with the pipe or snout material for the inner rings and a material that is consistent with the casing for the outer rings. The rings are made of a material whose surfaces form only thin oxide layers and that have a small coefficient of friction.
It has also been proposed to apply a high temperature dry film lubricant to the mating surfaces of the rings to reduce both the formation of oxides and to reduce the coefficient of friction.
Such arrangements are disclosed in U.S. Pat. No. 5,037,115, wherein it is stated: "With regard to the differential expansion coefficients identified above, the combination of initial clearance and differential growth must achieve a good seal for moderately low temperature, known to occur at start up, without incurring unsatisfactorily high stress and yielding or creeping at higher operating temperature", and
". . . a high temperature dry film lubricant is applied to the various sliding surfaces to further reduce friction and to further discourage the formation of oxide films on the ring, shell and pipe surfaces . . ." PA1 "This motion must take place without creating such high stress in the small rings as to cause permanent deformation and leakage. Leakage will occur if the rings become stretched and no longer seal against the pipe."
The normal operating temperature of a steam turbine will be about 1000 DEG F if the inner or outer rings are designed for a close fit and seal at say 500 DEG F, then when the turbine reaches its normal operating temperature of 1000 DEG F, one or all of the following may occur:
1) Since the inner and outer rings are fabricated from super alloy type material, it is reasonable to assume that if the rings are designed to seal at 500 DEG F, as stated above, then when the turbine reaches its normal operating temperature of 1000 DEG F an excessive amount of stress will occur in both the pipe or snout and the casing. This type of stress could cause either the snout or the casing to crack, causing severe damage to some or all of the turbine parts.
2) If cracking does not occur as stated above, then as stated in U.S. Pat. No. 5,037,115,
3) Another problem which is not accounted for in the above mentioned patent, or any other known prior art, is that the snout and the casing are not round at assembly, that is have not been machined to a true circle, as is the case in many instances.
Normal operating procedure during an overhaul is to remove the old rings and sand blast the mating surfaces, leaving a rough surface and not correcting any existing out-of-roundness. Such out-of-roundness of the snout or casing will allow for leakage.
4) The dry film lubricant coating is washed away by steam in a very short period of time.